Electrical scanning probe microscope apparatus

ABSTRACT

An electrical scanning probe microscope (SPM) apparatus. The SPM apparatus is equipped with an atomic force microscope with an infrared laser source, a position-sensitive photo-detector (PSPD) to provide a topographic image, a charge-coupled device (CCD) monitor for optical alignment, and an electrical scanning sensor operatively coupled to the atomic force microscope to acquire synchronous two-dimensional electrical images. The photoperturbation effects induced by stray light and perturbation of the contrast of SCM images can thus be ameliorated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation-in-part of application Ser. No. 10/861,385, filedon Jun. 7, 2004 now U.S. Pat. No. 6,975,129, the teachings of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an electrical scanning probe microscopeapparatus, and more particularly to an electrical scanning probemicroscope apparatus comprising an atomic force microscope equipped witha long-wavelength laser source.

2. Description of the Related Art

Scanning probe microscopes (SPMs) providing very high resolution imagesof various surface properties are typically employed as a means ofmeasuring surface topography and corresponding electrical propertyanalysis. Different types of electrical properties can be measured suchas, conductivity, voltage, capacitance, resistivity, current, andothers. Accordingly, many different SPM techniques may be used whenmeasuring electrical properties. For example, SPM techniques that may beused for synchronously providing electrical signals corresponding totopographic images comprise scanning capacitance microscopy (SCM),scanning spreading resistance microscopy (SSRM), Kelvin force microscopy(KFM) and conductive atomic force microscopy (C-AFM).

The scanning capacitance microscope (SCM) apparatus works by scanning atiny tip over the surface of a sample being imaged, while synchronouslymeasuring the electrical properties of the sample. A typical SCMapparatus comprises an atomic force microscope (AFM) and an ultra-highfrequency (UHF) resonant capacitance sensor can synchronously provide atwo-dimensional image. The AFM acquires surface topographic images, andthe UHF resonant capacitance sensor provides a synchronous twodimensional differential capacitance images. The AFM typically comprisesa cantilever and a conductive probe at the free end of the cantilever.In most AFMs the position of the cantilever is detected with opticaltechniques. A red laser beam (670 nm) reflected off the back of thecantilever onto a position-sensitive photo-detector is adapted to detectthe position of the cantilever. The AFM can thus generate topographicimages. However, photoperturbations, such as the photovoltaic effect andthe high-level carrier injection effect induced by stray light of theAFM red laser beam lead to distorted differential capacitance (dC/dV)profiles and hence perturb the contrast of SCM images.

According to recent research, narrow band-gap semiconductors, such asSi, GaAs, InP or others, suffer from the aforementionedphotoperturbations. The photoperturbations induced by the AFM laser beamnot only affect the image contrast of SCM images, but also reduce theaccuracy of the determination of the carrier concentration distribution.Solutions to these problems have been long sought but thus far haveeluded those skilled in the art.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide anelectrical scanning probe microscope apparatus comprising an atomicforce microscope equipped with a long-wavelength laser to overcome thephotoperturbation effects and improve the accuracy of junction images,in particular for ultra-shallow junctions in the narrower band-gapsemiconductors.

In order to achieve the above objects, the present invention provides anelectrical scanning probe microscope apparatus, comprising: an atomicforce microscope with an infrared laser source and a position-sensitivephoto-detector (PSPD) to provide a topographic image; a charge-coupleddevice (CCD) monitor for optical alignment; and an electrical sensordevice operatively coupled to the atomic force microscope to form asynchronous two-dimensional electrical image. The PSPD comprises anInGaAs or a Ge PIN photodiode.

In order to achieve the above objects, the present invention provides anelectrostatic force microscope apparatus, comprising: an atomic forcemicroscope with an infrared laser source and a position-sensitivephoto-detector (PSPD) to provide a topographic image; wherein asynchronous two-dimensional electrical potential image is acquiredcorresponding with the topographic image.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the office upon request and paymentof the necessary fee.

The present invention can be more fully understood by reading thesubsequent detailed description in conjunction with the examples andreferences made to the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an electrical scanning probemicroscope apparatus according to an embodiment of the presentinvention;

FIG. 2 is a partial schematic view of an electrical scanning probemicroscope apparatus using a long-wavelength laser beam as a surfaceimage measuring setup;

FIGS. 3A–3C are schematic views of an AFM laser beam aligned atdifferent points on the cantilever;

FIGS. 4A and 4B are differential capacitance (dC/dV) profiles of thelow-energy-BF₂ ⁺-implanted region with different AFM laser beam setupsshown in FIGS. 3B and 3C, respectively;

FIGS. 5A and 5B are two-dimensional differential capacitance (dC/dV)images of the low-energy-BF₂ ⁺-implanted region with different AFM laserbeam setups shown in FIGS. 3B and 3C, respectively;

FIGS. 6A and 6B are two-dimensional differential capacitance (dC/dV)images of a MOS device region with different AFM laser beam setups shownin FIGS. 3B and 3C, respectively; and

FIG. 7 is schematic view of a Kelvin force microscope (KFM) using along-wavelength laser beam as a surface potential image measuring setup.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described with respect to preferredembodiments in a specific context, namely in a scanning capacitancemicroscope (SCM) apparatus. The invention may also be applied, however,to other applications, such as scanning spreading resistance microscopes(SSRM), Kelvin force microscopes (KFM) and conductive atomic forcemicroscopes (C-AFM).

FIG. 1 is a functional block diagram of an electrical scanning probemicroscope (SPM) apparatus according to an embodiment of the presentinvention. The (SPM) apparatus comprises an atomic force microscope witha long-wavelength laser as a surface image measuring setup to provide atopographic image and an electrical scanning sensor device tosynchronously provide a two-dimensional electrical image. The electricalscanning sensor device comprises a capacitance sensor, a spreadingresistance sensor, or a current sensor. The atomic force microscope isequipped with a long-wavelength laser source as a surface morphologyimage measuring setup. The wavelength of the laser source exceeds 670nm, and more preferably equals 1.3 or 1.55 μm. The electrical scanningsensor device can provide two-dimensional differential capacitanceimages.

Referring to FIG. 1, an electrical scanning probe microscope (SPM)apparatus comprises a sample stage 100 for supporting a sample 200 andcontrolling the position of the sample 200. An AC biasing source 110 anda DC bias 120 are operatively coupled to the sample stage 100. Ascanning probe device comprises a cantilever 300 and a conductive probe320. The conductive probe 320 moves toward and away from the surface ofsample 200 with oscillatory motion, preferably at or near a resonantfrequency of the scanning probe device. Conductive probe 320 isoperatively coupled to an electrical sensor 500, such as a UHFcapacitance sensor, a spreading resistance sensor, or a current sensor.From electrical sensor 500, modulated capacitance signals pass tolock-in amplifier 600. Preferably, a UHF-resonant capacitance sensordetects the capacitance between conductive probe 320 and sample 200.Alternatively, a capacitive bridge circuit or an impedance transformermay be used instead of sensor 500.

In the most common configuration, a long-wavelength laser source 400,such as laser diode, generates a laser beam 402 which bounces off theback of the cantilever 300 onto a position-sensitive photo-detector(PSPD) 420, such as an InGaAs or a Ge PIN photodiode. As the cantilever300 bends, the position of the laser beam 402 on the detector 420shifts. A calibration device 440, such as a charge-coupled device (CCD)monitor, calibrates the position of the laser beam 402.

Lock-in amplifier 600 demodulates the capacitance signals at theoscillation frequency, or at some combination of frequency oscillationharmonics, of conductive probe 320, resulting in signals that correspondto the modulation amplitude of the probe-sample capacitance. Thesesignals pass to a controller 700, and then pass into an AFM feedbackcircuit 800 to be stored for each data point with respect to X and Yposition on sample 200. Such data may also be passed to a display device(not shown) for display as an image of sample capacitance or topography.The operations and functions of the AFM may be referred to U.S. Pat. No.6,127,506 which is fully incorporated by reference herein.

FIG. 2 is a partial schematic view of an AFM detecting the position ofthe cantilever with long-wavelength laser optical techniques. Along-wavelength, such as 1.3 or 1.55 μm, laser beam 402 bounces off theback of the cantilever 300 onto a position-sensitive photo-detector(PSPD) 420. The long-wavelength laser source 400 comprises laser diodewith excellent electro-optical performance equipped with athermo-electric cooler (TEC) to cool the operating temperature.

An operative embodiment is provided illustrating the influence of thephotoperturbations on measured differential capacitance (dC/dV) signalsfor a low-energy-BF₂ ⁺-implanted silicon substrate. A <100> n-typesilicon substrate is provided. A silicon oxide layer of 25 nm inthickness is thermally grown on the substrate. The substrate dopinglevel is approximately 5×10¹⁵ atom/cm³. The oxide layer is patternedwith standard photolithography and reactive ion etching processes toform a grating pattern. The grate and spacing widths are 0.8 and 2 μmrespectively. The resultant substrate is implanted with BF₂ ⁺ ions at anenergy of 20keV and ion dosage of 5×10¹⁴cm⁻². RTA treatments of theimplanted substrate are then performed at 1050° C. for 30 sec in an N₂ambient to form p⁺-n junctions. After RTA processes, a plasma-enhancedtetra-ethyl-ortho-silicate (TEOS) layer of 500 nm in thickness isdeposited on the substrate. A low-energy-BF₂ ⁺-implanted siliconsubstrate is thus formed.

In accordance with the present invention, the synchronous SCM images areacquired using a contact mode AFM. The cantilever of the conductiveprobe of the AFM is approximately 200–450 μm long, 30–50 μm wide and 5μm thick. The contact force of the conductive force is lower than ananonewton.

FIG. 3A is schematic view of an AFM laser beam aligned at differentpoints on the cantilever. The AFM laser beam is aligned on the tip site(setup 1) for acquiring the AFM image to confirm the surface conditionsand to obtain the cantilever height as a reference for later SCMmeasurements as shown in FIG. 3B. A first corresponding SCM image isacquired by setup 1. The AFM laser beam is aligned on the cantilever(setup 2) by using previously obtained cantilever height to producelower stray light intensity on the scan region as shown in FIG. 3C. Asecond corresponding SCM image is acquired by setup 2.

FIGS. 4A and 4B are dC/dV profiles of the low-energy-BF₂ ⁺-implantedregion acquired by different AFM laser beam setups. In FIG. 4A, thestray light illumination can generate carrier injection, therebyincreasing the effective carrier concentration of the scan region, andaccordingly reduce the SCM signals

$\left( {{i.e},{\frac{\mathbb{d}C}{\mathbb{d}V}{_{1}{< \frac{\mathbb{d}C}{\mathbb{d}V}}}_{2}}} \right)$The reduced SCM signals can cause lower contrast images. In accordancewith embodiments of the present invention, the scanning probe microscopeapparatus is equipped with a long-wavelength laser atomic forcemicroscope to overcome carrier injection and enhance the contrast ofdC/dV images of the low-energy-BF₂ ⁺-implanted region.

FIGS. 5A and 5B are two-dimensional dC/dV images of the low-energy-BF₂⁺-implanted region for different AFM laser beam setups. In FIG. 5A, thestray light absorption causes a photovoltaic effect at a junctionregion, thereby reducing the measured junction width W₁ to be less thanthe junction width W₂. The measured deviation induced byphotoperturbations can be up to approximately 50%. In accordance withthe present invention, the scanning probe microscope apparatus isequipped with a long-wavelength laser atomic force microscope toovercome the photovoltaic effect and acquires accurate two dimensionaldC/dV images of the low-energy-BF₂ ⁺-implanted region.

FIGS. 6A and 6B are two-dimensional dC/dV images of a MOS device regionfor different AFM laser beam setups. In FIG. 6A, a red laser beam withwavelength of 670 nm generates optical absorption and causes a surfacephotovoltaic effect and a carrier injection effect, thereby reducing themeasured effective channel length L₁ to less than the effective channellength L₂. The measured deviation induced by photoperturbations can beup to approximately 11.2%. In accordance with the present invention, thescanning probe microscope apparatus equipped with long-wavelength laseratomic force microscope to overcome the photoperturbation effects andacquires accurate effective channel length of the MOS device, as shownin FIG. 6B.

The present invention is illustratively exemplified as a scanningcapacitance microscope, although other electrical scanning probemicroscopes, such as scanning spreading resistance microscope (SSRM),Kelvin force microscope (KFM) and conductive atomic force microscope(C-AFM) can also be equipped with long wavelength laser AFM according tothe present invention.

Alternatively, non-contact mode scanning microscopes, such aselectrostatic force microscopy (EFM) or Kelvin force microscope (KFM)can also be equipped with a long wavelength laser source and a PINphotodiode sensor. FIG. 7 is schematic view of a Kelvin force microscope(KFM) using a long-wavelength laser beam as a surface potential imagemeasuring setup. Referring to FIG. 7, an electrostatic force microscope(EFM) apparatus comprises a sample stage 100 for supporting a sample 200and controlling position of the sample 200. A scanning probe devicecomprises a cantilever 300 and a conductive probe 320. An AC biasingsource 110 and a DC bias V_(tip) are operatively coupled to thecantilever 300. The conductive probe 320 moves toward and away from thesurface of sample 200 with oscillatory motion, preferably at or near aresonant frequency of the scanning probe device. The conductive probe320 is operatively coupled to AC biasing source 110. First, atopographic image 250 is taken. Then the probe 320 is retracted apredetermined distance, such as 200 nm from the sample 200 and apotential image 260 is made. The potential image 260 shows thevariations of the electrostatic potential in the same sample area.

In the most common configuration, a long-wavelength laser source 400,such as laser diode, generates a laser beam 402 which bounces off theback of the cantilever 300 onto a position-sensitive photo detector(PSPD) 420, such as a PIN photodiode sensor. As the cantilever 300bends, the position of the laser beam 402 on the detector 420 shifts. Acalibration device 440 calibrates the position of the laser beam 402.

Lock-in amplifier 600 demodulates the electrical potential signals atthe oscillation frequency, or at some combination of frequencyoscillation harmonics, of conductive probe 320, resulting in signalsthat correspond to the modulation amplitude of the probe-sampleelectrostatic force. These signals pass to a controller 700, and thenpass into an AFM feedback circuit 800 to be stored for each data pointwith respect to X and Y position on sample 200. Such data may also bepassed to a display device (not shown) for display as an image of sampleelectrical potential image or topographic image.

While the invention has been particularly shown and described withreference to preferred embodiments, it will be readily appreciated bythose of ordinary skill in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe invention. It is intended that the claims be interpreted to coverthe disclosed embodiment, those alternatives which have been discussedabove, and all equivalents thereto.

1. An electrical scanning probe microscope apparatus, comprising: anatomic force microscope with an infrared laser source and aposition-sensitive photo-detector (PSPD) to provide a topographic image;a charge-coupled device (CCD) monitor for optical alignment; and anelectrical sensor device operatively coupled to the atomic forcemicroscope to form a synchronous two-dimensional electrical imagewherein the atomic force microscope comprises: a sample stage; ascanning probe device comprising a cantilever and a conductive probe atthe free end of the cantilever; a topographic image device, operativelycoupled to the scanning probe device comprising the infrared lasersource, the PSPD corresponding to the infrared laser source, and acalibration device for calibrating the infrared laser source; and acontroller for controlling the position of the scanning probe device. 2.The electrical scanning probe microscope apparatus as claimed in claim1, wherein the PSPD comprises an InGaAs or a Ge PIN photodiode.
 3. Theelectrical scanning probe microscope apparatus as claimed in claim 1,wherein the wavelength of the infrared laser source is approximately 1.3μm or 1.55 μm.
 4. The electrical scanning probe microscope apparatus asclaimed in claim 1, wherein the electrical scanning sensor devicecomprises a capacitance sensor, a spreading resistance sensor, or acurrent sensor.
 5. An electrostatic force microscope apparatus,comprising: an atomic force microscope with an infrared laser source anda position-sensitive photo-detector (PSPD) to provide a topographicimage; and a charge-coupled device (CCD) monitor for optical alignment;wherein a two-dimensional electrical potential image is acquiredcorresponding to the topographic image, and wherein the atomic forcemicroscope comprises: a sample stage; a scanning probe device comprisinga cantilever and a conductive probe at the free end of the cantilever; atopographic image device, operatively coupled to the scanning probedevice comprising the infrared laser source, the PSPD corresponding tothe infrared laser source, and a calibration device for calibrating theinfrared laser source; and a controller for controlling the position ofthe scanning probe device.
 6. The electrical scanning probe microscopeapparatus as claimed in claim 5, wherein the PSPD comprises an InGaAs ora Ge PIN photodiode.
 7. The electrostatic force microscope apparatus asclaimed in claim 5, wherein the wavelength of the infrared laser sourceis approximately 1.3 μm or 1.55 μm.